Typically, gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine blade assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine vane and blade assemblies, to these high temperatures. As a result, turbine airfoils, such as turbine vanes and blades must be made of materials capable of withstanding such high temperatures. In addition, turbine airfoils often contain internal cooling systems for prolonging the life of the airfoils and reducing the likelihood of failure as a result of excessive temperatures.
Typically, turbine airfoils, such as turbine vanes are formed from an elongated portion having one end configured to be coupled to an outer shroud vane carrier and an opposite end configured to be movably coupled to an inner shroud. The airfoil is ordinarily composed of a leading edge, a trailing edge, a suction side, and a pressure side. The inner aspects of most turbine vanes typically contain an intricate maze of cooling circuits forming a cooling system. The cooling circuits in the vanes receive air from the compressor of the turbine engine and pass the air through the ends of the vane adapted to be coupled to the vane carrier. The cooling circuits often include multiple flow paths that are designed to remove heat from the turbine vane. At least some of the air passing through these cooling circuits is exhausted through orifices in the leading edge, trailing edge, suction side, and pressure side of the vane.
Composite airfoils have been developed for use in turbine engines as composite materials are typically suitable to use in higher temperature environments that conventional metals forming airfoils. Composite airfoils are often constructed as laminate layers formed from high strength fibers woven into a cloth that is saturated with a ceramic matrix material. The multiple laminate layers are stacked, compacted to the desired thickness, dried, and fired to achieve the desired structural properties. The laminates have desirable in-plane structural properties but significantly less strength in the through plane direction. The composite airfoils are often formed from an inner solid core, a laminate layer, and a FGI insulating thermal barrier coating. A ceramic bond exists between the laminate and solid core and at the interface of the laminate and thermal barrier coating.
The composite airfoils have been cooled in conventional composite airfoils by passing cooling air from a compressor through the airfoil. Typically, the cooling fluids are passed through a plurality of cooling channels and exhausted through the trailing edge of the composite airfoil without use of film cooling. While outer surfaces of composite airfoils are typically exposed to temperatures of about 1,600 degrees Celsius in a turbine engine, the laminate layer of the airfoil is generally kept at a temperature less than about 1,100 degrees Celsius. Typically, cooling air used in a composite airfoil cooling system is about 450 degrees Celsius. The extreme temperature gradient between the combustion gases at 1,600 degrees Celsius outside of the airfoil and the cooling gases at 450 degrees Celsius in the interior cooling channels creates thermal stress in the composite airfoil that can delaminate the laminate layer and destroy bonds between the laminate layer and the inner core and between the laminate layer and the thermal barrier coating. Such problems with thermal stress do not exist in metal airfoils because of the high thermal conductivity of the metal forming the airfoil and high strength of the metal. Thus, a need exists for reducing the thermal stresses created by cooling fluids in composite airfoils in turbine engines.